IMO the article misses the point there. Immutability is the "thing" in FP.
There is no doubt in my mind that the Purity concept in Haskell is taken to an impractical degree. I happen to like it, but it does not make developing software with it any less of a troubled experience. The defining concept in comparing Haskell and Clojure for me is the pareto principle :D Clojure gets 80+% of the way there, without 80% of the self flagellation.
There is also a consideration of static typing vs dynamic but that's a whole other can of worms.
Simply, in Haskell all mutability is delegated to the runtime while in clojure one has the freedom and responsibility to manage it directly, and boy does it gain in simplicity for it!
I know I am preaching to the choir but it's pleasant to find a similar experience and share thoughts!
You have this certainty to the extent that the compiler enforces it.
If you like language X because it's 'pragmatic' instead of 'religious', then you make the pragmatic choice of not knowing with certainty.